Market value over quality?

[Tactician]

It's not worthy to buy young, expensive 3* players. Maybe they are fast trainers but you need A LOT of greens to develop them. Even a 4* player, maybe is better but still needs a lot of sources.
But in lv1 there are some free agents, young expensive & fast trainers. Those are worthy to buy them, train them and sell them for good money.

Yes, but it depends. If the player plays well and contributes to bringing good results, he is worth it, especially if the player is about to become a 4 star (needs only one point in overall quality). What such players maybe not worth for is for the long term; when you are promoted to the next level you will want to start with a better rated player. But still, I prefer a hard working player who plays consistently well and is 3-4 stars rather than a lazy 5 star that brings mostly ratings of 5-6.

I agree though that a lot of green packs (and may be red packs) will be needed to make a 3 star fast trainer to 5+ stars, especially if the player is not on the point of becoming a 4 star. But, if he is playing frequently, getting good ratings and even some M.O.T.M, he will probably reach 5+ stars quicker and with less green packs.